Cellulosic structure and method for preparing same



March 3, 1942. w. H. CHARCH ET AL v 2,275,347

CELLULOSIC STRUCTURE AND METHOD FOR PREPARING SAME Original Filed Oct. 25, 1935 flrllllllllllllll".

AT TO RN Y.

- several diierent thicknesses.

Patented Mar. 3, 1942 CELLULOS-IC STRUCTURE lAND METHOD FOR PREPARING SAME falo, N. Y., assignors,

Nemours & Company, Wil- E. I. du Pont de William Hale Charcli and Francis P. Alles, Bufby mesne assignments, to

mington, Del., a corporation of Delaware Application October 23, .1935, Serial No. 46,324

, Renewed March 2, 1940 11 Claims.

regenerated cellulose sheeting cast from viscose. However, it is to be understood that the invention is applicable to the production of cellulosic sheets and films from aqueous alkaline cellulosic solutions generally. y

Heretofore, in the manufacture of such sheets and lms from viscose, films have been cast in The finished sheet produced for ordinary purposes is about 0.001

of an inch thick although thinner or `thicker sheets have been produced for other and special purposes.

Although such films have satisfactory tensile strength, for certain purposes they do not have the required degree of toughness to satisfactorily resist the shock and handling to which they are ordinarily subjected in commercial applications. The tensile strength of a nlm is measured by the magnitude of the force necessary to rupture it,

and depends upon the cohesive forces existing within the structure of the film. The toughness of a film is measured by the length of time it will resist rupture under actual handling conditions, and depends primarily upon its elongation or the degree to which it will yield to a force without breaking. These two factors are clearly distinguishable. For example, a film of as high tensile strength as it has hitherto been possible to produce may lack toughness in that it will fail to yield suniciently to the shocks encountered in commercial use to avoid premature breaking, cracking or splitting.

- In the films of the prior art it has been customary to impart a certain amount of toughness or durability, together with fiexibility, by incorporating therein a softener, such as glycerin or ethylene glycol. Besides the question of expense, the use of these materials is attended with certain diiiiculties. The film vtends to become sticky, and various precautions. are therefore necessary in order to avoid caking of film in rolls or stacks or sheets. For example, nlm con-v taining above l4% glycerin must usually be sized or provided with atenuous coating of a suitable substance to avoid this sticking. At still higher glycerin contents the sizing must be so thick that it become a true coating. Additionally, the

lm becomes so soft that it is extremely diflicult if not impossible to handle by means of automatic machinery. Furthermore, the high hygroscopicity of these softeners tends to cause the film to absorb more water than it would in their absence and therefore to vary more widely in dimensions with the relative humidity of the atmosphere.' Finally, the use of liquid softeners detracts 'somewhat from of the film. As these objectionable characteristicsA vary directly with-the amount of glycerin or similarsoftener used, a point will be reached where it`is no longer practically possible to increase the softener content. It has been found to be of advantage for some uses to increase the toughness without increasing the softener content and for other uses to secure the advantages of a lower softener content while still retaining s the desired degree of flexibility and toughness or durability. 1

It is therefore an object of this invention to produce non-fibrous sheets and films, having good transparency [and flexibility, and of greatly increased durability or toughness compared to films of similar softener content hitherto known.

It is a further object of this invention to produce non-fibrous sheets and films having the desired degree of flexibility, durability and toughness, with lower amounts of softener.

It is a still further object to produce such ysheets and films from regenerated cellulose.

Other objects of the invention will appear hereinafter.

TheA objects of this invention are in general accomplished by conducting the preliminary drying step with a substantially complete absence of.

tension.

In one method of manufacture of regenerated cellulose sheets and films a Viscose solution is forcedthrough an elongated orificeA into a coagulating bath. The freshly coagulated film is then led through successive treating-baths, such as regenerating, Washing, desulfurng, bleaching and glycerinating baths to a drying apparatus.

, found in accordance with the present invention that non-fibrous cellulosic sheets and films much tougher per unit of thickness than known heretofore and atthe same time retaining good transparency and being free of Wrinkles and puckers can be produced by substantially eliminating ten* sion during the first portion of the drying operation and drying with normal, increased or rethe strength or tenacity ducd tension thereafter. Furthermore, sheets and lms produced in this manner will exhibit the desired degree of flexibility and toughness with lower amounts of softener. It has been foundrin accordance with this invention that,

contrary to the process used in making the ordiin the lower moisture range complete absence of tension will produce puckers and wrinkles and will impair the transparency of the nlm. Tension, less than normally used in accordance with the prior art, can, of course, be employed in drying below this moisture content, if desired, but tension cannot be completely eliminated.

'I'he preferred form of apparatus for `carrying out the present invention is substantially the same as that shown in U. S. Patent No. 2,099,162 (Eberlin). 'Ihis apparatus (described in the said Eberlin patent Vas applicable to the production of all types of film) should be modied so as to achieve this shrinkage and should dry the film without tension down to the required moisture content. It should be emphasized that tension should be substantially eliminated in all directions. For example, in operating the Eberlin drier to produce the ordinary lm, shrinkage is substantially completely prevented in the longitudinal direction or direction of travel of the lm, due to the fact that the film, after traversing the Eberlin drier, is traveling at substantially as high a rate of speed as before entering said drier. When operating in accordance with the present invention, however, the Eberlin drier must be operated in such a manner that the lm within the drier is entirely free to shrink in all directions.

The details of the invention will be more clearly apparent by reference to the following description taken in -connection with the accompanying drawing illustrating one embodiment of apparatus suitable for carrying out the present invention and in which the figure is a fragmentary diagrammatic elevational View of a drying apparatus.

Referring now to the drawing, the nlm Io is led through the nal treating tank II, usually containing a softener which impregnates the film, over rotating rollers I2 and squeeze rollers I3 and I4, into the preliminary drying apparatus indicated generally 'at I5. This 'consists of an enclosure containing a series of jets I8 directing streams of hot air or open gas flames against the sides of the film. Positioned adjacent and below the path of travel of the film may be placed nickel rods 21 in order to support the film and keep it away from the immediate vicinity of the burners. If desired, the jets I8 positioned above the lm may be omitted, in which case it may be desired to omit the nickel rods 21.

After emerging from the preliminary .drying apparatus I5, the film is led over a roller I6, which travels at a lower peripheral rate of speed than rollers I2, I3 and I 4. The rate should be so adjusted that ythe lm, during its passage through the drying chamber I5, is under substantially no tension and is therefore allowed to shrink freely in all directions. At the same time,

of course, the roller ly slow that the lm is allowed to pile up in the chamber I5. The film should be so constantly withdrawn from the chamber I5 that substantially the same amount of film remains therein at all times.

The rate of travel through the .chamber I5 and the amount and temperature of the heat supplied should be such that the moisture content of the film is reduced to the required amount (preferably `between 100% and 200%) in the substantially complete absence of tension. It is obvious that these factors can be appropriately controlled by one skilled in the art.

After being led over roller I6 the film travels over a series of the well known drier rollers Ml, the speed of which, together with that of roller IB, is so adjusted that tension, whether normal, reduced or elevated, is applied to the film from this point down to the point at which it is completely dry. This is necessary in order to secure lm of good transparency and to avoid the formation of puckers and Wrinkles.

In order to test the toughness of various types of lm under actual handling conditions, the following test has been devised. A sample of the film to be tested is formed into a bag, and 200 grams of dry white -beans inserted therein. The bag is Iclosed so as to leave the lm tightly wrapping the beans, without loose fla-ps, and sealed by means of a piece of pressuresensitive -regenerated cellulose adhesive tape. The bag is then. repeatedly dropped from 'a height of two feet onto a glass plate, in an atmosphere of 35% relative humidity. The number of times that it can be dropped without :breaking is a direct measure of its toughness, and may be considered, for the purpose of this specification, as the toughness factor.

The following table indicates the results of such a test, an average of 24 samples being taken in each i It will be seen that film processed according to this invention is much tougher than film produced in accordance with the .prior art with the same amount of softener, the improvement in lall cases being at least 25%, and in most cases far in excess of this.

Films or pellicles prepared in accordance with the present invention and containing normal amounts of a softener such as glycerin exhibit greatly improved durability or `toughness over similar tener, shown by the above table. the glycerin content is the same, they also exhibit increased freedom Ifrom stickiness. For example, regenerated cellulose lm prepared in ac present invention and concordance with the taining 14% glycerin will not exhibit objectionable sticking for approximately seventeen ldays at relative humidity and one poundper I6 should not be so excessive- I sion, and completing lm, and still retain the required degree of toughness. The advantages of such a procedure are, as pointed out above, increased freedom from 4stickiness, less susceptibility to changes in atmospheric moisture, and greater tenacity. The` cost factor. in using lower quantities of glycerin is obvious.

In all cases, in addition to the above advantages, iilm prepared according to'this invention exhibits freedom from wrinkles and puckers and no impairment of transparency.

The invention, although particularly useful for the production of regenerated cellulose sheets 'and films from viscose, is also applicable to the production of sheets and films from aqueous alkaline cellulosic solutions coagulable in an acid coagulating bath includingfor example, regen-4 erated cellulose produced from cuprammonium cellulose and lowly etheried or lowly esterifled cellulose. The invention finds its greatest use in its application to films and sheets of gel regenerated cellulose, that is, cellulosic films which have been coagulated and/or regenerated, washed, bleached, desulfured and glycerinated but not yet dried. However, improved results can also be obtained in treating sheets and films which have been previously dried, by rewetting the same and redrying them in accordance with the present invention. i

Many changes and modifications of the invention as above specifically set forth may obviously be made without departing from the nature and spirit of the invention.' It is therefore to be unsaid sheets and lms derstood that the invention not to be limited except as set forth in We claim: l

1. In the method of' producing non-fibrous, cellulosic sheets and films from aqueous alkaline cellulosic solutions the steps comprising coagulating said sheets and lms, purifying the same, preliminarily drying the same to a point at which they contain between 100% and 200% moisture based on the weight of the cellulosic material, with substantially complete elimination of-tenthe drying of said films the appended claims.

under tension.

2. Inthe method of toughening non-fibrous, cellulosic sheets and films cast from aqueous alkaline cellulosic solutions, the steps which comprise passing said films through a bath containing a softening agent, preliminarily drying the same to a point at which they contain between 100% and 200% moisture based on the weight of the cellulosic material, with substantially complete elimination of tension, and completing the drying of said films under tension.

3. In the methodv of toughening regenerated cellulose. sheets -and films, the steps which-comprise passing said films through a bath. containing a softening agent, same to a point at which they contain between 100% and 200% moisture based on the weight of regenerated cellulose,l with substantially complete elimination of tension, and completing the drying of said films under tension.

4. In the method of toughening regenerated cellulose sheets and iilmsthe steps which compreliminarily drying theprise passing ing glycerin, preliminarily drying the same to a point at which they contain between 100% and 200% moisture based on the weight of regenerated cellulose, with substantially complete elimination of tension, and completing the drying of said films undertension.

5. In the method of producing non-fibrous, cellulosic sheets and films from aqueous` alkaline cellulosic solutions, the steps comprising coagulating said sheets and films, purifying the same, preliminarily drying the same, with substantially complete elimination of tension, until the moisture content has been reduced to such an extent that further drying under tension will result in an improvement in toughness of the said sheets and lms over that which will result in the absence'of said preliminary drying step, said sheets and films after said preliminary drying step containing at least 100% moisture based on the weight of the cellulosic material, and completing the drying of said films under tension.

6. In the method of toughening regenerated cellulose sheets and films, the steps which comprise passing said films through a bath containing glycerin, preliminarily drying the same, with substantially complete elimination of tension, until the moisture content has been reduced to such an extent that further drying under tension will result in an improvement in toughnessof the said sheets and films over that which will result in the absence of said preliminary drying step,

after said preliminary drying step containing at least 100% moisture based on the weight of regenerated cellulose, and completing the drying of said films under tension.

'7. In the method of vtoughening non-fibrous, cellulosic sheets and films cast from aqueous alkaline cellulosic solutions, the steps which comprise passing said films through a bath containing a softening agent, preliminarily drying the same*J with substantially complete elimination of tension, until the moisture content has been reduced to such an extent that further drying under tension willresult in an improvement in toughness Aof the said heets and lms over that which will result in the absence ofl said preliminary drying step, said sheets and iilmsl after said preliminary drying step containing at least 100% moisture based on the weight of the cellulosic material, and completing the vdrying of said nlms under tension. 8. The process which comprises passing continuous gel regenerated cellulose web through a drier in substantially complete absence of tension in the web, until the water content has been reduced to 200% to 100% based on the weight 'J of the cellulose material whereby the web is permitted to shrink freely, and thereafter drying while preventing shrinkage of the web.

9. In the method of toughening regenerated cellulose sheets and films, the steps which comprise preliminarily drying wet regeneratedcellulose sheets and films to a point at which they containv between and 200% moisture, based on the weight 'of regenerated cellulose, with substantially. completing thedrying of said films under tension. v

10. In the method of toughening regenerated cellulose sheets and films, the steps which comprise passing said films ing a softening agent, preliminarily drying the same, with substantially complete elimination of 'l5 tension, until the moisture content has been resaid lms through a bath containcomplete elimination of tension, and` through a bath containi duced to such an extent that further drying ated cellulose, and completing the drying of said under tension will result in an improvement in lms under tension.

toughness of the said sheets and lms over that 11. The process of claim 1 when the cellulosic which will result in the absence of said prelimsheet is a lowly etheried cellulose.

nary drying step, said sheets and films after 5 said preliminary drying step containing at least WILLIAM HALE CHARCH.

100% moisture based on the weight of regener- FRANCIS P. ALLES. 

